Nexys4 DDR™ FPGA Board Reference Manual
Copyright Digilent, Inc. All rights reserved.
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Two on-board LEDs (LD23 = LED2, LD24 = LED1) connected to the PHY provide link status and data activity
feedback. See the PHY datasheet for details.
EDK-based designs can access the PHY using either the axi_ethernetlite (AXI EthernetLite) IP core or the
axi_ethernet (Tri Mode Ethernet MAC) IP core. A mii_to_rmii core (Ethernet PHY MII to Reduced MII) needs to be
inserted to convert the MAC interface from MII to RMII. Also, a 50 MHz clock needs to be generated for the
mii_to_rmii core and the CLKIN pin of the external PHY. To account for skew introduced by the mii_to_rmii core,
generate each clock individually, with the external PHY clock having a 45 degree phase shift relative to the
mii_to_rmii Ref_Clk. An EDK demonstration project that properly uses the Ethernet PHY can be found on the
Nexys4 DDR product page at
www.digilentinc.com
.
ISE designs can use the IP Core Generator wizard to create an Ethernet MAC controller IP core.
NOTE: Refer to the LAN8720A data sheet
4
for further information.
C11
C9
A9
Artix-7
B3
RESET#
INT#/REFCLK0
CRS_DV/MODE2
TXEN
MDIO
4
MDC
D10
B8
D9
RXD1/MODE1
TXD0
SMSC LAN8720A
RJ-45 with
magnetics
Link/Status
LEDs (x2)
TXD1
RXD0/MODE0
RXERR/PHYAD0
CLKIN
D5
B9
A8
A10
C10
Figure 5. Pin connections between the Artix-7 and the Ethernet PHY.
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Oscillators/Clocks
The Nexys4 DDR board includes a single 100 MHz crystal oscillator connected to pin E3 (E3 is a MRCC input on bank
35). The input clock can drive MMCMs or PLLs to generate clocks of various frequencies and with known phase
relationships that may be needed throughout a design. Some rules restrict which MMCMs and PLLs may be driven
by the 100 MHz input clock. For a full description of these rules and of the capabilities of the Artix-7 clocking
resources, refer to the “7 Series FPGAs Clocking Resources User Guide” available from Xilinx.
Xilinx offers the Clocking Wizard IP core to help users generate the different clocks required for a specific design.
This wizard will properly instantiate the needed MMCMs and PLLs based on the desired frequencies and phase
relationships specified by the user. The wizard will then output an easy-to-use wrapper component around these
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http://ww1.microchip.com/downloads/en/DeviceDoc/8720a.pdf
